1. Field of the Invention
This invention relates to the construction of multisensor radiation detector systems designed for removable insertion into a nuclear flux region such as found in nuclear reactors. Such systems may be referred to as detector assemblies.
2. Description of the Prior Art
Radiation detection devices of various configurations which are intended for insertion and use within the core of a nuclear reactor are well known in the prior art. The various designs may be typified by those shown in the U.S. Pat. No. 4,087,693, to Brown, et al., issued May 2, 1978; the U.S. Pat. No. 3,879,612, to Foster, et al., issued Apr. 22, 1975; and the U.S. Pat. No. 3,375,370, to Hilborn, issued Mar. 26, 1968. The radiation detection devices illustrated in these various patents include the type known as self-powered detectors wherein external power is not required. In such a detector an emitter, a collector, and an insulator material between the two are used to generate an electric current which is indicative of the intensity of the radiation present at the location of the emitter element as detailed in the Russian publication titled "Energy Transformation of Short-Life Radio Active Isotopes", by M. G. Mitelman, R. S. Erofeev and N. D. Rosenbloom, 1960. The U.S. patent to Hilborn is relevant for its disclosure of a self-powered neutron detector assembly. The U.S. Patent to Foster, et al, is relevant for its disclosure of a multi-sensor radiation detector system. That patent shows the use of a self-powered detector in combination with what is referred to in that patent as a fission chamber. The detector and the fission chamber are connected electrically in parallel requiring but two conductors extending out of the reactor to an external electrical circuit. Switching means are employed to switch from the detector to the fission chamber. The U.S. Pat. No. 4,087,693 to Brown, et al., discloses the use of silicon dioxide as a dielectric for insulation of the emitter element.
While the various prior art radiation detection systems each has its particular advantages, typically all suffer from a common disadvantage resulting from its small diameter (typically 1/16 inch) and the long length of the detector. The typical radiation detector may range in length from 30 feet to over 100 feet. In order to insure accurate measurement of radiation levels, the manufacturing process must be carefully controlled. The detector element used in such detectors normally comprises a lead wire, which may be of a metal such as stainless steel, at the end of which is affixed an emitter element, typically of rhodium. The rhodium emitter and lead wire are insulated from an outer metallic conductor by a material which is typically aluminum oxide or magnesium oxide. The assembly comprising the central lead member, the emitter element, the insulating material and the outer metallic conductor is then swagged so as to compact the insulation material. The insulation is typically in the form of short cylinders of aluminum oxide or magnesium oxide slipped over the center wire and meant to be crushed around the wire during the swagging process. Since these cylinders are hard ceramics, the swagging can nick and break the lead member resulting in a high rejection ratio of sensors. The swagging and other steps in the production process may result in the central lead element and the rhodium emitter element deviating along their length from the center line of the outer conductor. This deviation from the center line may result in inaccuracies in measurements of radiation levels. Also such deviation from center would result in a different thickness of insulation material being present between the rhodium emitter element and the source of the radiation. As the thickness of the insulator material increases, the percentage of the charged particles emitted by the detector which is absorbed by the insulator material also increases. Charged particles so absorbed do not reach the outer conductor and thus are not measured. These inaccuracies in axial alignment of the emitter element can contribute to inaccuracy in measurement of radiation levels.
One method of assuring the centering of the center conductor and rhodium emitter element along the axis of the outer conductor element is that disclosed in U.S. Pat. No. 4,087,693. That method comprises the use of a silicon dioxide insulation which is initially in the form of a cloth or woven material which has been formed into a loose sock and placed over the emitter element and the lead wire. This assembly is then inserted into a length of metal tubing forming a sheath which is subsequently drawn through a sizing die. It is stated in that patent that at high levels of compaction the silicon dioxide fibers are easily broken since they are relatively brittle and the broken pieces move or flow with some preference for axial alignment as the outer sheath is drawn through a die. The fragmented insulation then flows easily around the lead wire and the rhodium emitter element and centers it along the length of the detector. This method would appear to be suitable for centering a single lead wire and rhodium element within a single outer conductor. However, such a method would not insure proper centering and alignment of multiple rhodium emitter elements and lead wires as well as proper spacing of the multiple detectors throughout the length of a long outer sheath.
In prior art detector systems the sensors are individually calibrated and then assembled into a multi-sensor detector; the individual sensors can not thereafter be calibrated. To insure accuracy of measurement, each sensor must be calibrated prior to assembly.
It is thus an object of the invention to provide an easily installed, high yield, relatively flexible device for assuring accurate and consistent location of an emitter with respect to the center line of an outer sheath along the full length of the active radiation detecting zone.
It is a further object of the invention to provide a multiple sensor detector system which accurately accounts for background radiation and does not require calibration of the sensors.
Another object of the invention is to provide enhanced reliability through the absence of nicks and fractures of the emitters and lead wires.